The present invention relates generally to semiconductor device processing techniques and, more particularly, to an improved method for forming silicide contacts on semiconductor devices using cobalt as the deposited metal.
In the manufacture of semiconductor devices, salicide (or self-aligned silicide) materials are formed upon gate conductors and diffusion regions to reduce the line resistance of a CMOS device, thereby improving the speed characteristics thereof. In salicide technology, a refractory metal or a near noble metal, such as titanium for example, is deposited on a silicon substrate. The deposited metal is then annealed, thereby forming a silicide layer only on the exposed areas of the substrate. The areas of unreacted metal left on the dielectric may then be selectively etched away without a masking step. Thus, the process is “self-aligning.”
As circuit devices have continued to shrink in size, however, it has been found that titanium silicide (TiSi2) becomes an unsatisfactory silicide material since the sheet resistance thereof begins to sharply increase when the linewidth of the device decreases below 0.20 μm. More recently, cobalt disilicide (CoSi2) has been used as a replacement for titanium in salicide structures since it does not suffer from a linewidth dependent sheet resistance problem. On the other hand, the use of cobalt silicide structures is not without its own drawbacks. For example, unlike titanium, a cobalt layer requires a cap layer such as titanium nitride (TiN) due to the sensitivity of cobalt to contaminants during the annealing process.
In addition, the quality control of silicide contacts in general becomes an increasingly difficult problem with smaller dimensions and more complex material mixtures. For instance, silicide growth may be non-uniform due to preferred growth along certain crystal planes or different levels of defect density due to implant damage or from silicon regrowth following anneal sequences. Accordingly, it would be desirable to be able to improve upon the manner in which the cobalt/silicon interface is initially formed, so as to improve the quality of the cobalt monosilicide and, subsequently, the cobalt disilicide. This would, in turn, improve the etch resistance and thermal degradation of the silicide material.